test: inspect manifests in manifest-only jobs

We don't build ppc64le and s390x in gitlab CI, we only generate the
manifests to ensure there's no issues with manifest generation and
package selection.  However, we can also validate them using 'osbuild
--inspect' to make sure they're at least valid manifests.

Signed-off-by: Achilleas Koutsou <achilleas@koutsou.net>

test/scripts/configure-generators

@@ -100,7 +100,14 @@ generate-manifests-{distro}-{arch}:
     INTERNAL_NETWORK: "true"
   script:
     - sudo ./test/scripts/install-dependencies
-    - go run ./cmd/gen-manifests --arches {arch} --distros {distro} --workers 10
+    - go run ./cmd/gen-manifests --arches {arch} --distros {distro} --workers 10 --metadata=false --output ./manifests
+    - for manifest in ./manifests/*; do
+        if osbuild --inspect $manifest > output; then
+          echo "$manifest OK";
+        else
+          cat output;
+        fi;
+      done
 """